1. Field of the Invention
The present invention relates to a driving circuit of a liquid crystal display (LCD) device, and more particularly to a driving circuit of a reflective or transflective LCD device.
2. Discussion of the Related Art
Flat panel display (FPD) devices have been the subject of recent research due to their small size, light weight, and low power consumption. Among FPD devices, LCD devices are most widely used because of their excellent resolution, color display range, and other display quality characteristics. LCD devices typically comprise first and second substrates, wherein each substrate supports respective electrodes that face each other, and a liquid crystal layer is interposed between the first and second substrates. Due to an electric field generated by a voltage applied to the respective electrodes, the liquid crystal layer exhibits optical anisotropy. Using optical transmittance differences defined by the optical anisotropy, LCD devices may be used to display images.
In general, LCD devices include driving devices to drive the liquid crystal layer interposed between first and second substrates.
LCD devices are non-emissive display devices and therefore require a light source. Depending on whether the display device requires an internal or external light source, LCD devices may be defined as transmissive LCD devices and reflective LCD devices, respectively.
Transmissive LCD devices include an LCD panel and an internal light source provided as a backlight device. By selectively adjusting an alignment of the liquid crystal layer, the LCD panel may display images by selectively adjusting the transmittance of light emitted by the backlight through the LCD panel. Accordingly, the first and second substrates are transparent substrates and the respective electrodes may be formed of transparent conductive material. Transmissive LCD devices are capable of displaying bright images in darkened environments due to the presence of the backlight, however, power consumption of the transmissive LCD is increased due to operation of the backlight device.
Reflective LCD devices include a first substrate that supports a first electrode formed of transparent conductive material to allow for the passage of the ambient light, and a second electrode formed of conductive material of high reflectance. By selectively adjusting the alignment of the liquid crystal layer, as discussed above, the LCD panel may display images by selectively adjusting the transmittance of ambient or artificial external reflected light. Since reflective LCD devices use external or ambient light to display images, power consumption characteristics of reflective LCD devices are relatively low compared with the that of transmissive LCD devices. However, reflective LCD devices are not easily viewed in darkened environments.
Due to the limitations of the transmissive and reflective LCD devices described above, transflective LCD devices, capable of being selectively viewed in either of the aforementioned transmissive or reflective modes at the user's discretion, are currently the subject of research and development.
FIG. 1 illustrates a schematic plan view of an LCD device with a driving device.
Referring to FIG. 1, an LCD panel 10 includes an array substrate 11 and a color filter substrate 12. A liquid crystal layer (not shown) is interposed between the array and color filter substrates 11 and 12, respectively. Since the array substrate 11 has a larger area than the color filter substrate 12, a portion of the array substrate left is left uncovered by the color filter substrate 12. This uncovered portion supports a pad (not shown) that is used for applying a signal to a line of the LCD panel 10. The pad is connected to a tape carrier package (TCP) 30 including gate and source driving integrated circuits (ICs) 31 used for driving the LCD panel 10. The TCP 30 is also connected to a printed circuit board 20 (PCB), on which a plurality of devices are formed, and from which various control and data signals are generated. The TCP 30 is formed in a packaging method that connects the driving ICs 31 to the LCD panel 10. The TCP 30 may include a flexible film capable of being bent towards a rear surface of the LCD panel 10, with a driving IC 31 mounted thereon. Since the driving ICs are mounted on the flexible film, the LCD device may be made compactly. Alternatively, the driving ICs may be connected to the LCD panel 10 using either chip on glass (COG) or chip on film (COF) methods. Using the COG method, the driving ICs are mounted to the array substrate 11 and the volume of the LCD device increases relative to the volume of the LCD device employing TCP. Similar to the TCP method, COF methods mount driving ICs to an extra film, thereby creating a compact structure. Accordingly, TCP or the COF methods are typically used over COG methods.
The array substrate 11 includes a pixel electrode and a TFT for applying a signal to the pixel electrode. The color filter substrate 12 includes a color filter layer and a common electrode. The pixel electrode of the array substrate 11 includes a liquid crystal capacitor connected to the common electrode of the color filter substrate 12. A storage capacitor may be connected to the liquid crystal capacitor to maintain an applied voltage until a subsequent signal is applied. Accordingly, a leakage current between the pixel and common electrodes may be reduced when a voltage is applied to the liquid crystal capacitor. Storage capacitors further provide other advantages such as increasing gray level stability, reducing flicker, and reducing residual images.
FIG. 2 illustrates an equivalent circuit diagram of one pixel of an LCD device having a storage capacitor.
Referring to FIG. 2, a pixel including a TFT 53, a liquid crystal capacitor 54 (CLC), and a storage capacitor 55 (Cst) may be defined at crossings of gate and data lines 51 and 52, respectively. The TFT 53 includes gate “G”, source “S” and drain “D” terminals connected to gate and data lines 51 and 52, respectively. The TFT 53 switches data signals applied to the liquid crystal capacitor 54 on or off. The liquid crystal capacitor 54 and the storage capacitor 55 are connected parallel to each other and used as loads. A parasitic capacitance 56 (Cgs) is generated between the gate “G” and the source “S”.
In a normal driving state, when “high” signals are applied to the gate “G”, a channel of the TFT 53 is opened between the source “S” and the drain “D”. Therefore, charge to and discharge from the liquid crystal capacitor 54 and the storage capacitor 55 may be performed through the source “S” and the drain “D”. When power to the LCD device is turned off, power is not supplied to the gate “G” and the channel is closed. When the channel is closed, load charges are not discharged through the channel but are gradually discharged through the parasitic capacitance between the gate “G” and the source “S” and a leakage current of the channel. Therefore, undesirable residual images may remain long periods of time, even after power to the LCD is turned off. For transmissive LCD devices, residual images are not displayed because power supplied to the backlight device is also turned off. However, for reflective or transflective LCD devices, residual images remain because reflective LCD and transflective LCD devices use ambient light as a light source.
FIGS. 3A and 3B illustrate plan views of a reflective LCD device before and after a power is off, respectively. The reflective LCD device exhibits a normally white mode, in which a white is displayed when a voltage is not applied.
Referring to FIGS. 3A and 3B, discharge occurs from the center of the LCD panel after power to the LCD device is turned off and radiates to the edges of the LCD panel. Accordingly, image erasure originates at the center of the LCD panel and radiates towards the edges of the panel. When power of reflective or transflective LCD devices is turned off, the channel of a TFT is closed by the gate and residual charges of a LCD panel are not discharged. Accordingly, undesirable residual images remain.